1. Field of the Invention
This invention relates to an image forming apparatus using electrophotography and in particular to an image forming apparatus comprising a developing unit using a magnetic developer.
2. Description of the Related Art
A one-path color print system for printing two colors or more in one cycle is available as one system of electrophotography. As shown in JP-A-48-37148, the following developing method is available: A photosensitive body is charged and then the light exposure amount is changed in three levels of no exposure, weak exposure, and strong exposure in response to color information of an image on exposure to light, whereby electrostatic images at three levels of a high-level area, a low-level area, and an intermediate-level area therebetween as surface potential on the photosensitive body are formed and the high-level area is normally developed and the low-level area is inversely developed for forming two types of toner images on the photosensitive body. Toners different in charge polarity can be used as the two types of toners. A two-color image can be formed by using toners different in color as in the example in the related art. The description to follow assumes a two-color developing method for forming a two-color image.
As the electrostatic image developing method, a method using a dual-component developer of a mixture of toner of colored particles and carrier of magnetic particles is available. To develop the high-level area, a developing bias voltage between the high-level surface potential and the intermediate-level surface potential is applied to a developing electrode of a developing unit and on the other hand, to develop the low-level area, a developing bias voltage between the low-level surface potential and the intermediate-level surface potential is applied to the developing electrode of the developing unit, whereby the toners are developed in the high-level area and the low-level area.
FIG. 2 shows a schematic configuration of a printer using a system wherein three values of potential on a photosensitive body are provided, described in JP-A-48-37148 as one of the one-path color print systems. Throughout the specification, the system is called potential division developing system. FIG. 3 is a potential model chart of the potential division developing system. The system is a system for printing in two colors of black and any other color than black, such as red, blue, or green. In the description to follow, any other color than black is red and the first developing is normal developing in a red developer and the second developing is inverse developing in a black developer.
A photosensitive body 1 having photoconductivity is charged by a charger 2 and then exposure responsive to image information is executed by an exposure unit 3. A black image portion is exposed to strong light so that the photosensitive potential becomes the minimum as potential VL, a print area in red is not exposed to light as potential VH, and a background portion, namely, a white paper portion is exposed to weak light as potential VM of about a half the potential VH, whereby three-value potentials are formed. Next, a first developing unit 4a develops the red image portion. The developing unit 4a contains a developing roller 30a having a rotatable sleeve roller 31a on the outer periphery of a magnet 32a internally fixed. The sleeve roller 31a is electrically connected to a power supply and a bias voltage Vb1 is applied so that |VH| greater than |Vb1|xe2x89xa6|VM|. A dual-component developer 20a of a mixture of a red toner 21a in the first developing unit 4a and a first carrier 25a for charging the red toner 21a to an opposite polarity to the charge polarity of the photosensitive body 1 by friction with the red toner 21a at an appropriate ratio is magnetically attracted to the surface of the sleeve roller 31a and is transported to a developing area 40a with rotation of the sleeve roller 31a and scrubs the surface of the photosensitive body 1. The red toner 21a is deposited on the red image area by an electric field formed by an electrostatic latent image formed on the photosensitive body 1 and the bias voltage Vb1 applied to the sleeve roller 31a, and developing is executed.
Next, a second developing unit 4b develops the black image portion. A black developer 20b used here is a mixture of a black toner 21b and a second carrier 25b for charging the black toner 21b to the same polarity as the charge polarity of the photosensitive body 1 by friction with the black toner 21b at an appropriate mix ratio. A bias voltage Vb2 applied to the sleeve roller 31b is set so that |VM| greater than |Vb2| greater than |VL|, and the black image portion is developed.
An image is thus formed in the black and red toners different in charge polarity. Thus, in the state, if an attempt is made to apply a corona of a single positive or negative polarity and transfer in a transfer unit 7, the toner of the same polarity as the applied corona repels and is not transferred to a sheet member 8. Therefore, before the toner is transferred, the polarity of the toner needs to be matched with the polarity opposite to that of the applied corona in the transfer unit 7, and a before-transfer charger 6 is installed. The toner with the polarity matched by the before-transfer charger 6 is transferred efficiently to the sheet member 8 by the transfer unit 7. After the toner is transferred, the sheet member 8 passes through a fuser 9 and the toner is fixed onto the sheet member 8.
The remaining toner or a deposit of paper powder, etc., on the photosensitive body 1 after transfer section passage is removed and collected from the photosensitive body 1 by a cleaning unit 11.
The potential division developing system for performing two-color print according to the described process has very excellent features that it can execute two-color print at the same speed as monochrome print, that the costs are low because of one exposure unit, and that a position shift in a two-color image does not occur.
However, in the developing methods using a dual-component developer as well as the above-described two-color developing method, the carrier is developed in the potential area opposite to the potential area in which toner is developed with respect to the developing bias voltage; this is a problem. The reason is that in the dual-component developer, the toner and the carrier are mixed, whereby they are charged in opposite polarities to each other. For example, in the above-described two-color developing method, as shown in FIG. 3, to develop the high-level area charged negatively, the developing bias voltage Vb1 between the high-level surface potential VH and the intermediate-level surface potential VM is applied to the developing electrode of the developing unit, whereby the toner charged to the positive polarity is developed in the high-level area. On the other hand, the carrier is charged to the negative polarity and thus is developed in the low-level area where the surface potential is lower than the developing bias Vb1. The carrier is magnetic particles and the developing electrode of the developing unit generally is a developing roll having an internal magnet wherein an external metal cylinder rotates, and the carrier is made to stay by the magnetic force of the internal magnet so that it is not developed to the photosensitive body. However, it is hard to completely make the carrier stay, and slight carrier developing occurs.
A phenomenon in which the carrier is developed as mentioned above generally is called carrier deposition, and also raises a problem of causing a toner image transfer failure to occur in the transfer step of a post-step of developing in one-color image formation in the related art. As a measure against the carrier deposition, after developing, a carrier collection roll having an internal magnet wherein an external metal cylinder rotates like the developing roll of the developing unit is placed and the deposited carrier is attracted by a magnetic force and the external metal cylinder is rotated for collecting the carrier in the developing unit, whereby the problem is solved.
However, as shown in FIG. 3, the photo sensitive body charge potential is divided into two areas and latent images of a two-color image are prepared and thus the developing contrast potential of each color (|VH-Vb1| in red, |Vb2-VL| in black) is also reduced by half as compared with a usual single-color print system. Thus, it is difficult to provide a high print density; this is a problem. To enlarge the contrast potential, the photosensitive body charge potential may be raised. However, there is a limit from the point of the life of the photosensitive body and the difference between the developing bias potential and the image portion potential of another color (|Vb-1-VL| in red, |VH-Vb 2| in black) is also enlarged inevitably and thus in the area, the electric field oriented for depositing the carrier on the photosensitive body becomes very strong. Therefore, there is a problem of a drastic increase in carrier deposition on the photosensitive body as compared with the usual single-color print system. A harmful effect caused by the carrier deposition in the usual single-color print system is a transfer failure of a toner image in the proximity of deposition of carrier produced by impairing intimate contact between a sheet member and the photosensitive body at the transfer time because of the deposited carrier on the photosensitive body and the carrier-deposited area is a white paper area and thus if the carrier is somewhat deposited, a defect does not appear on the print image.
In the potential division developing system, however, the carrier-deposited area is an area in which another toner image is formed and thus if a carrier is deposited only a little, accordingly toner is not deposited on the toner image and a background is exposed, namely, a defect called a white dot occurs on the final print image.
This is occurrence of the following problem: In the two-color image forming apparatus, when a high-level area is developed, the carrier of the developer is deposited on a low-level area and in addition, the potential of the low-level area is disturbed. This is a problem revealed from the two experiment results of the fact that when a high-level area is developed as the first color and then a low-level area is developed, a large number of white dots occur in the image in the low-level area and the fact that when only a low-level area of the second color is developed without developing the first color as high-level area, low-level area, and intermediate-level area are formed as surface potential of photosensitive body, white dots do not occur. That is, the carrier comes in contact with the photosensitive body, whereby the potential of the low-level area is raised and the portion is not developed in the second color and appears as a white dot.
This means that the developing unit adopting the potential division developing system requires suppressing carrier deposition on the photosensitive body and executing high-density print more than the usual single-color print system in stricter restrictions as compared with the single-color print system.
As a developing system for enhancing the print density, a system using a plurality of developing rolls, a system using a low-resistance developer called conductive magnetic brush developing, and the like can be used alone or in combination. However, the system using a plurality of developing rolls inevitably involves upsizing the developing unit and an increase in costs and is hard to use with a small-sized developing unit, and the conductive magnetic brush developing involves a problem of resolution, a problem of the developer life, etc. In a system in which a latent image portion developed by a second developing unit is scrubbed by a first developing unit like the above-described potential division developing system, if the conductive magnetic brush developing is used for the first developing unit, it is feared that the second latent image may be disturbed at the first developing time.
That is, in the above-described configuration, the following problem easily occurs: At the first red image developing time, the potential VL at which the black toner is deposited rises, the print density of the black image is lowered, resolution degradation occurs, or the potential VM of the background portion becomes non-uniform and fogging increases.
Therefore, as the developing unit used with the potential division developing system, a high developing capability and less carrier deposition become important and in the developing unit used for the first developing, it also becomes important not to disturb the latent image formed on the photosensitive body.
As a small-sized developing system being capable of providing a high print density and having a weak force of scrubbing a photosensitive body, a system of placing magnetic poles of the same polarity in an entrance part and an exit part of a developing area using a magroller called W magnetic roller, etc., is known.
FIG. 11 is a schematic drawing to show one example of the system. A first developing pole 27 is placed in an entrance part of a developing area 40 and a second developing pole 28 of the same polarity as the first developing pole 27 is placed in an exit part of the developing area 40. A developer 20 first forms a magnetic brush in the developing area entrance part and scrubs a photosensitive body 1, then is moved toward the second developing pole 28. At the point in time, the magnetic binding force acting on the developer 20 is weakened by a repulsive magnetic field, no magnetic brush is formed, the developer comes apart, a large number of clouds occur, and developing is performed. Next, the developer is magnetically bound by the second developing pole 28 and again magnetic brush developing is performed. That is, in this method, developing is performed in one developing area in the order of a portion of the entrance part with a comparatively strong magnetic field, a portion of the center with a comparatively weak magnetic field, and a portion of the exit part with a comparatively strong magnetic field. Since the developing efficiency is raised because of a toner cloud occurring in the weak magnetic field portion of the center, the print density is enhanced and moreover no magnetic brush is formed in the center of the developing area and the magnetic field is comparatively weak even in the magnetic brush formation area. Thus, the scrubbing force of the photosensitive body is weakened as compared with the method in the related art for forming a strong magnetic field in the whole developing area.
However, in the center portion, the carrier is easily deposited on the photosensitive body and the force of collecting the deposited carrier in the exit portion is also weakened and thus there is a problem of an increase in carrier deposition on the photosensitive body.
Aside from the above-described problem, there is a problem of thinning the density of the rear end part of a solid image. In the periphery of the solid image, toner receives the action of being pulled into a latent image because of the peripheral effect of enhancing the electric field in the periphery of the latent image. Particularly, in the rear end part of the solid image portion, the direction in which the developer scrubs the photosensitive body with rotation of the developing roll is the same as the direction in which the peripheral effect acts on the toner, so that the toner deposited on the rear end part is scraped into the solid image. Thus, the problem of thinning the density of the rear end part of the solid image occurs.
In the example in the related art, the above-described problem occurs and thus the image density does not become uniform and a clear image cannot be provided.
It is therefore an object of the invention to provide a small-sized and low-cost developing unit capable of executing high-density print even at a low contrast potential and having a high resolution without carrier deposition, particularly a developing unit for providing sufficient performance even in a system wherein carrier deposition easily occurs and the detriment effect of the deposited carrier easily appears, such as the potential division developing system. It is another object of the invention to provide an image forming apparatus capable of providing a clear image with occurrence of no white dots and having a uniform density.
The first problem is solved by using a developing unit comprising a fixed magnet having a magnetic pattern such that the magnetic flux densities of the entrance and exit portions of a developing area in which a magnetic developer comes in contact with a photosensitive body are high and that the magnetic flux density of the center of the developing area is low and a sleeve roller being placed on the outer periphery of the magnet for rotation, wherein the magnetic developer is transported with rotation of the sleeve roller and an electrostatic latent image formed on the photosensitive body is scrubbed with the magnetic developer for developing, wherein the maximum magnetic flux density of a first area in which the magnetic flux density is high in the entrance portion of the developing area is 500 Gauss or more on the surface of the sleeve roller, wherein the maximum magnetic flux density of a second area in which the magnetic flux density is high in the exit portion of the developing area is 700 Gauss or more on the surface of the sleeve roller, wherein the minimum magnetic flux density of an area in which the magnetic flux density is low in the center of the developing area on the surface of the sleeve roller is 60% or less of the maximum magnetic flux density of the exit portion of the developing area, and wherein the first and second areas in which the magnetic flux density is high are formed and are placed facing the photosensitive body so that the angle between a center point when points at which the magnetic flux density becomes 80% of the first maximum magnetic flux density on both sides of point at which the first maximum magnetic flux density is reached on the surface of the sleeve roller are connected and a center point of a line connecting points at which the magnetic flux density becomes 80% of the second maximum magnetic flux density on both sides of point at which the second maximum magnetic flux density is reached becomes 25 degrees or more.
The second problem is solved by using a developer whose electric resistance is lowered small if the applied voltage is raised is used as the developer for forming the first toner image.
The third problem is solved by using a developer whose electric resistance is lowered if the applied voltage is raised is used as the developer for forming the second toner image.